This invention relates to a brake system having anti-skid control and/or traction slip control including a pedal-operated braking pressure generator and an auxiliary-force-generating system by means of which controllable auxiliary forces can be produced which either assist or oppose the pedal force. Wheel sensors and electronic circuits are provided for detecting the rotational behavior of the wheels and for generating auxiliary force control signals.
An anti-skid-controlled brake system of this type is disclosed in the German Published Patent Application No. DE-OS 33 17 629 (Corresponding to U.S. Pat. No. 4,702,531). That system includes a master cylinder with a servo arrangement connected upstream. By metering in a vacuum or auxiliary hydraulic pressure, an auxiliary force is generated by the arrangement. The auxiliary force either assists or opposes the pedal actuating force. Moreover, multi-directional valves are inserted into the pressure medium lines leading from the master cylinder to the wheel brakes, which multi-directional valves can be changed over to lock. If the associated electronics detect a lock-up tendency at one or more wheels, the master cylinder pressure, and thus the braking pressure, is reduced due to the build-up of an auxiliary force opposed to the pedal force. The pressure build-up becomes effective only at the wheels that have become instable. At the same time, due to locking of the pressure medium paths leading to the remaining wheels, the pressure medium is enclosed therein and thus the braking pressure is kept constant. By means of the multi-directional valves, the individual wheel brakes are consecutively connected to the master cylinder each time the auxiliary force is varied so such as to ensure that the desired pressure level will be established in the connected wheel.